Load post content in background thread

[bummytime]

Original issue: https://github.com/wordpress-mobile/WordPress-Android/issues/586

Copy the entire contents of Romeo and Juliet into a post: http://shakespeare.mit.edu/romeo_juliet/full.html

The app will become sluggish as it loads the content into the post editor. If you save the post as a local draft, the view post fragment also struggles to load the content. We should load the content in background thread in order to not block the UI.

[maxme]

Webview loads data in its own thread, so there is no risk of a non responsive app when opening the visual editor.

This problem still exists when switching from visual to html mode.